1. Field of the Invention
The present invention relates to a centerless grinding machine of the type wherein a regulating wheel cooperates with a work blade for rotatably carrying and driving a workpiece during grinding operations.
2. Description of the Prior Art
Generally, in centerless grinding machines, the machining accuracy depends largely upon a center height angle (.gamma.) indicated in FIG. 3. The angle (.gamma.) amounts to the sum of angles (.alpha.) and (.beta.) which a line extending across the axes of a grinding wheel 11 and a workpiece W and a line extending across the axes of a regulating wheel 13 and the workpiece W respectively make with a line extending across the axes of the grinding wheel 11 and the regulating wheel 13. It is known that a high precision grinding can be realized where the center height angle (.gamma.) is set to around seven (7) degrees. The center height angle (.gamma.) is determined by the workpiece finish diameter, the grinding wheel diameter, the regulating wheel diameter and a center height (the distance from the workpiece axis to the line extending across the axes of the wheels in a direction normal to the line) and varies as the diameters of the wheels are decreased by dressings thereon.
In order to avoid such variation of the center height angle, it has been a common practice to effect manual adjustment of the height position of the work blade periodically in known centerless grinding machines. However, such manual adjustment requires a highly skilled operator because the adjusting amount is determined based upon the operator's experiences. Accordingly, in the known machines, it is difficult to maintain the machining accuracy of the workpieces high and particularly, to maintain the roundness of the workpieces constant and high.
Further, since the workpiece W is ground with its axis being offset normally from the line extending across the axes of the regulating wheel 13 and the grinding wheel 11 as shown in FIG. 3, the moving direction of the grinding wheel 11 is different from a direction in which the line across the grinding wheel axis and the workpiece axis extends, and likewise, the moving direction of the regulating wheel 13 is different from a direction in which the line across the regulating wheel axis and the workpiece axis extends. Thus, if the position of each of the wheels is compensated in the moving direction thereof by an amount which coincides with a decrease in radius of each wheel, a minute error in the finished workpiece dimension is cumulated each time of such dressing, whereby a precise machining accuracy cannot be maintained in a dead-stop plunge grinding mode.
In order to obviate this drawback, the positions of the grinding wheel and the regulating wheel are manually adjusted in the known centerless grinding machines each time the cumulated error attains a certain value. However, because the value of such error per dressing varies depending upon the diameters of the grinding wheel and the regulating wheel, precise compensation is difficult to perform manually. Accordingly, in the known centerless grinding machines, it is also difficult to machine workpieces precisely in the dead-stop mode.